Plasma display panels are classified into direct current (DC) type and alternating current (AC) type. The AC type PDPs are superior to the DC type PDPs in terms of luminance, light emitting efficiency, and length of life, and widely have been used.
In the AC type PDPs, an electrode and a dielectric layer are formed in this order on a front substrate, and a protective film further is formed to cover the dielectric layer. Magnesium oxide (MgO) is used as the material for the protective film. This is because magnesium oxide has been considered to be superior to other materials in terms of functions required for the protective film, that is, sputtering resistance and electron emission characteristics. It is possible to lower a discharge starting voltage of a PDP by using a material, such as magnesium oxide, having a large secondary electron emission coefficient (γ) for the protective film facing discharge spaces in the PDP.
JP 2000-173476 A (Patent Literature 1) proposes that a surface layer of a protective film be composed of a magnesium oxide in which oxygen is partly substituted by nitrogen. According to the Patent Literature 1, a PDP with a protective film having a surface layer with a composition represented by Mg3O3(1−x)N2x, where 0<x<7, has a lower discharge starting voltage than that of a PDP with a protective film composed of magnesium oxide.
JP 2003-100217 A (Patent Literature 2) proposes that a protective film have a composition represented by AlNX, where X is at least one selected from Si, Ge, Sn, Pb, Be, Mg, Ca, O, and S. According to the Patent Literature 2, AlN has an excellent sputtering resistance and electron emission characteristics, and adding an element other than Al and N thereto further enhances these properties (paragraphs 0022 and 0023). The Patent Literature 2 discloses, in Example 4 thereof, (Al1−a−bMaDb)1−d(N1−cAc)d, where M is at least one selected from Si, Ge, Sn, and Pb, D is at least one selected from Be, Mg, and Ca, A is at least one selected from O and S, and 0.3<d<0.5; “δ” in the Patent Literature 2 is rewritten as “d” here; and referring to claim 2, 0≦a≦0.5, 0≦b≦0.5, 0≦a+b≦0.5, 0≦c≦0.5, and 0<a+b+c≦1, although this is not clearly stated in the Example 4. In Table 4 showing the results of the Example 4, (AlMg)0.60(NO)0.40 is listed as an example.
As mentioned above, there conventionally have been proposed protective films in which the ratio of the number of nonmetal atoms, such as N and O, to the number of metal atoms, such as Mg and Al, is less than 1 (for example, the ratio is 2/3 in the above-mentioned (AlMg)0.60(NO)0.40). This seems to be related to the well-known fact that the secondary electron emission coefficient is increased when magnesium oxide is made oxygen-deficient, as disclosed in paragraph 0005 of the Patent Literature 1.